This proposal is aimed at expanding our knowledge of the psychological and physiological bases of language comprehension, primarily from scalp recordings of event-related brain potentials (ERPs). Current evidence suggests that the N400 component of the ERP is a physiological index of some aspect of word recognition reflecting the influence of semantic context at both the lexical and sentential levels. A late positive component of the ERP has been tied to the processes of memory encoding and retrieval, perhaps at a general level which encompasses both linguistic and extralinguistic knowledge. The relationship between the N400 and late positive components, and the psychological/physiological events underlying them, is underspecified at present and clarifying these relationships is also a goal of the proposal. Experiments 1 and 2 examine ERPs elicited by spoken words to clarify the interactions between lexical and sentence level semantics, the importance of word frequency in word recognition, and the timecourse of word recognition. Experiments 3 through 6 investigate the impact of other cognitive processes on language processing. Experiment 3 investigates the modification and reorganization of lexical knowledge by determining whether or not new or weakly-learned words produce different context effects than well-known words. Experiment 4 also investigates the structure of lexical knowledge and its relationship to memory retrieval processes by examining both semantic and phonological priming effects when subjects are in a tip- of-the-tongue state. Experiment 5 examines crosstalk between attended and unattended words under two different varieties of attention selection based on spatial location or color. This experiment will describe the relationship between the early attention-sensitive components of the ERP, thought to reflect filtering of unattended stimuli, and the later N400 component. Experiment 6 examines mnemonic processes during discourse comprehension in an attempt to better characterize the relationship between the late positive component and episodic learning or retrieval.